At the end of the charging process or when a battery is overcharged, oxygen is continuously released at the nickel positive electrode of a zinc nickel secondary battery. Oxygen recombination, the re-absorption of the oxygen released due to battery operation, occurs when the zinc oxidizes. It can also occur during the electrochemical reduction reaction at the surface of the negative electrode. In a conventional zinc-nickel secondary battery, the ability of the zinc negative electrode to recombine oxygen is poor. Therefore, at the end of the charging cycle or when the battery is overcharged, especially when the battery is charged with a high current, the oxygen generated cannot be adequately recombined at the zinc negative electrode. When the oxygen generation rate at the positive electrode exceeds the rate of oxygen recombination at the negative electrode, oxygen accumulation will cause the internal pressure of the battery to increase. This may result in the release of the safety valve and electrolyte leakage.
In some applications, the capacity for oxygen recombination has to be high as batteries are constantly subject to overcharging. For example, batteries in cordless telephones are generally charged whenever they are not in use. Therefore, the battery packs for these applications must be capable of withstanding overcharging without significant increase in internal pressure that may result in electrolyte leakage, erosion of the circuitry, and subsequent damage to the equipment.
There are two approaches to reducing the internal pressure of a battery. One is to decrease the oxygen generation during overcharging by increasing the over-potential for the oxygen evolution. The second is to increase the recombination of the oxygen at the zinc negative electrode.
In prior art, such as that disclosed in the Chinese Patent CN1501530A, to improve electrical conductivity, electrically conductive additives such as carbon black and graphite are added to the active material for the negative electrode. To improve the capacity for oxygen recombination for the zinc negative electrode, hydrophobic binding agent such as polytetrafluoroethylene (PTFE) are used to form a hydrophobic network, that, to a limited extent, creates pores within the negative electrode. These pores form pathways for the oxygen to enter into the electrode where additional oxygen recombination can occur at the internal surfaces of the pores.
However, this method is insufficient to adequately control the increase in internal pressure during overcharging as both the hydrophilic zinc oxide and the carbon black granules that have a large pore volume are highly liquid-absorbent. The customary process for fabricating zinc negative electrodes uses aqueous solutions. When zinc oxide, carbon black, metal oxide additives, and hydrophilic binding agents are stirred and mixed to form a paste during fabrication, the pores in the carbon black adsorb the hydrophilic binding agents such that the electrolyte fills the pores of the carbon black when it is injected into the battery. The filled carbon black pores cannot be utilized to facilitate oxygen recombination. Moreover, the electrolyte in the pores blocks and prevents oxygen from entering into the electrode to recombine.
Currently, to improve oxygen recombination at the zinc negative electrode, a split stacked negative electrode assembly design such as that described in U.S. Pat. No. 5,460,899 is widely used. In that design, an integrated negative electrode is formed with a porous hydrophobic membrane that separates two zinc electrode half-plates. The oxygen generated in the charging process can reach and recombine inside the zinc electrode half-plate through the porous hydrophobic membrane. Therefore, this hydrophobic membrane effectively doubles the area of the interface between the oxygen and the zinc negative electrode.
However, there are many disadvantages to this design as well. The use of two zinc electrode half-plates doubles the quantity of current collector needed and increases the cost of raw materials. Moreover, the process for fabricating an integrated zinc negative electrode is complicated and the labor costs for its fabrication are increased. In addition, this design is only suitable for laminated battery structures, usually for rectangular batteries. They are not suitable for winding type battery structures, usually for cylindrical batteries.
Due to the limitations of the prior art, it is therefore desirable to have novel negative electrodes and novel methods for fabricating negative electrodes that are low in labor and production costs, that is applicable for use in different battery designs, and that allow for increase oxygen recombination.